Determining if a power supply is isolated?

Question

I've been looking for a low cost adapter I can use to power some electronic circuits I'm playing with. Pretty much the only requirement I have at this time is that the adapter is mains isolated for my safety. However, I can't find any markings on the adapters I have or any adapters I've found online which would indicate if the adapter is mains isolated.

Is there a way to determine if an AC-DC wall adapter is isolated from mains, preferably without taking it apart, or are they all isolated and I'm making a large fuss over nothing?

A few different specific examples I had in mind:

• A cell phone USB charger
• Advertised as Linear unregulated AC-DC supply
• Advertised as a switch mode AC-DC supply
• A laptop power brick
• Advertised as a regulated supply (no indication if switch-mode or linear)

Answer 1

The older ones with substantial weight to them all contain transformers, so they will certainly be correctly isolated.

Most of the newer lightweight ones also contain small high-frequency transformers, so they also ought to be correctly isolated; however I would have my doubts about the very cheapest and possibly counterfeit "iphone charger" type of product.

If everybody is playing by the rules, anything with the "double-insulated" symbol (two nested square boxes) will be correctly isolated. Buy anything with this symbol from a reputable supplier and you should be good.

(For completeness : the mains isolation is usually "broken" by some small capacitors for noise suppression : these will (excepting counterfeits) be rated to high safety standards (Class Y or X2) and not considered to break the isolation for safety purposes; however there may be applications in e.g. sensitive medical electronics where even the tiny AC current they permit is too much. For all our sakes, I hope you're not asking about such applications on StackExchange! :-)